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Full text of "Handbook Of Chemical Engineering - I"

GRADING AND SCREENING
241
At the first passage one grain is removed and 99 remain. At the passage of the second line of holes 0.99 of a grain is removed which is estimated as one whole grain. At the third passage 0.98 grain is removed, also reckoned as a whole one, etc.
&',. f*f	First aperture		Second aperture		Third aperture		Fourth  aperture		Fifth aperture	
1Z6 OI grain, juilli-		Grains		Grains		Grains		Grains		Grains
metsrs	Grains	re-	Grains	re-	Grains	re-	Grains	re-	Grains	re-
	through	main-	through	main-	through	main-	through	main-	through	main-
		ing		ing		ing		ing		ing
9	1	99	1	98	1.	97	1	96	1	95
8	4	96	4	92	4	88	4	84	3	81
7	9	91	8	83	7	76	7	69	6	61
6	16	84	13	71	11	60	10	50	8	42
5	25	75	19	56	14	42	11	31	8	23
4	36	64	23	41	15	26	9	17	6	11
3	49	51	25	26	13	13	6	7	3	4
2	64	36	23	13	8	5	3	2	1	1
1	81	19	15	4	3	1	1	0	0	0
The significance of this partial tabulation (complete tabulation for the elimination of all the grains being too lengthy) is better understood when the weight efficiency figures are computed. It is assumed that the grains are all of the same kind of material and that consequently the volume computations can be used to determine the weights and percentages. The efficiencies by weight after passing the five apertures are in round figures successively:
PERCENTAGE
After passing first aperture.......         8
second aperture.....        14
third aperture......        19
fourth aperture.....        23
fifth aperture.......        26
The quickness of the elimination of the small grains will be noted and also the slowness of the large grains which carry the most weight.
Commercial Screening.—The theory shows the difficulty of eliminating grains of nearly the size of the aperture and the tendency of sizes smaller than the apertures to pass into the oversizes. In the majority of the rock-crushing plants of the East and the ore-crushing plants of the West, the screening work will average to an efficiency of about 60 per cent. The various sizes produced by batteries of screens in the plants average about 40 per cent of undersize material.
In the most precise commercial grading, the best work being done in some plants in the abrasives industry, the efficiency of the screening does not actually run much over 75 per cent. As shown by hand screens the work is better than this, in some cases over 90 per cent. Hand screens as a means of testing the work of power screens are only better in degree than power screens. With hand test screens of over J^-in. size it is possible after shaking out finer particles to pick up the coarse pieces of the test sample and try them in different axial position to see if they will pass the apertures.
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